| Maraging steel(18Ni300)is a kind of high-strength steel with the characteristics of small heat treatment deformation,good processing and welding performance.And extremely high strength and hardness can be obtained after solution and aging treatment.So it has been widely used in precision mould,aerospace,missiles,aircraft landing gear and other fields.Selective Laser Melting(SLM)is one of the most potential additive manufacturing technologies,which has the advantages of high manufacuturing freedom and manufacturing efficiency,making it rapidly applied in direct manufacturing of personalized,customized functional parts with high precision and complex structure.Maraging steel manufacured by SLM has excellent fine grain structure,low hardness(380HV),so it is easy to be machined.Therefore,maraging steel functional parts with complex structure can be obtained by SLM technology first.Then milling and other traditional processing are used to improve the accuracy.Finally parts with hardness up to 650 HV are obtained by aging treatment.However,systemic study of maraging steel direct formed by SLM is insufficent at present.Few studies on typical structural features and precision manufacturing.And there is none application of high-performance maraging steel parts manufactured by SLM.Based on the above situation,this research focuses on the basic technology,properties controllability,structural characteristics and preliminary research on the application of high-precision maraging steel manufactured by SLM.The main content and conclusions are as follows:(1)Mechanism and process of maraging steel manfuactured by SLM.The single-pass scanning,fusion welding and interlayer fusion experiments were carried out respectively,then the melting characteristics of maraging steel manufactured by SLM and the evolution mechanism of melt morphology and block surface morphology were summarized.The mechanism of microstructural formation of the maraging steel manufactured by SLM and the influence of the manufacturing direction,traditional solution treatment and aging treatment on microstructure,hardness,tensile properties and impact properties were studied.The mechanism of microstructural formation of the maraging steel formed by SLM and the influence of the forming direction,traditional solution treatment and aging treatment on microstructure,hardness,tensile properties and impact properties were studied.Based on the above experimental results,the mechanism of laser-substance interaction in the SLM process was analyzed,and the physical processes of endothermic,melting,and solidification of the metal powder were discussed.(2)Precision optimization and machining of maraging steel manfuactured by SLM.The factors affecting the accuracy during data processing,printing,and post-processing in the entire process are analyzed,and spot compensation and design compensation methods were used to reduce the adverse effects of data processing and spot size on the dimensional accuracy of the manufacturing.The BP neural network and genetic algorithm are used to optimize the process,and the dimensional deviation in the X direction is within 0.087 mm,the dimensional deviation in the Y direction is within 0.129 mm,and the surface roughness is within 6.427 with optimized process parameters.The effect of heat treatment on the dimensional accuracy was studied,it was found that the overall size of the sample after heat treatment slightly increased,the maximum expansion rate after direct aging treatment was 0.18%,and the maximum expansion rate after solid solution and aging treatment was 0.26%.The influence of milling process on the surface roughness of maraging steel by SLM was studied,it was found that the higher the density,the better the surface quality after milling,and the surface roughness after milling was reduced to 0.98.Milling depth and forming direction have little effect on the surface roughness after milling,and it was found that the heat treatment state has little effect on the surface roughness of the milled sample by testing surface roughness of milling sample after heat treatment.(3)Manufacturing ability of typical feature structure.Based on the maraging steel complex functional parts,the typical geometric features such as thin walls,gaps,cylindrical square columns,square holes of circular holes,and inclination angles were extracted by concentrating,concluding,and summarizing.Manufacturing capacity of extreme size feature structures was investigated,surface topography and dimensional accuracy of feature structures were evaluated,and the constraints that restrict the formation of feature structures were analyzed,which was provide design guidelines and guidance for maraging grade functional parts manufactured by SLM.(4)The heat treatment process and properties controllability of maraging steel by SLM.Based on the DSC characteristics of maraging steel powder and its SLM manufacturing parts,heat treatment process plan was designed.Due to the rapid cooling characteristics of the SLM process,the direct manufacturing parts consist of a large amount of martensite and a small amount of austenite,which provides a good matrix for aging treatment.Therefore,the aging treatment of the SLM manufactured parts was directly studied,and the effects of aging temperature and time on the microstructure,phase and mechanical properties were studied.As a comparison,the microstructure and properties at different aging temperatures and times were studied,it was found that the direct aging treatment not only inherited the fine structure of the SLM direct-molded parts,in addition,the same mechanical properties as solid solution and aging can be obtained,so the solution treatment can be omitted and the aging treatment can be directly performed.(5)Design and direct manufacturing of precision mould with conformal cooling channel by SLM.The thermal expansion and thermal conductivity of maraging steel were studied,it was found that the thermal expansion performance of different maraging steels was basically the same in the low temperature range.The direct coefficient after the aging was 23.8W/m·K,which was slightly lower than the solution and aging treatment.After 25.7W/m·K.The surface structure characteristics of the cooling channel are extracted,it was found that the forming position has little effect on the shape accuracy of the channel,and the angle of the channel arc has a great influence on the forming quality.When the channel is completely horizontal,the shape accuracy is the worst,the rate exceeded 22%.Cooling channels were designed to inject mold cores,and compared with conventional baffle-type straight channel,the cooling efficiency was increased by about 10%.Three-dimensional inverse scanning technology was used to study the dimensional accuracy of maraging steel direct formed by SLM and parts after milling,it was found that die size deviation after molding between-0.23-+0.27 mm,and the dimensional accuracy is significantly improved after solution treatment and milling,the overall size is smaller by 0.1mm.In addition,the roughness and surface hardness of the mold before and after the milling and before and after the heat treatment were tested,and the roughness of the entire surface after milling was reduced to 1?m or less,the hardness remained basically unchanged. |
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